I have similar issues with my mom, and for this and other reasons we only visit her once about every few months, for about two or so hours at a time. It's a big concern for me now because my 12yo has become very self conscious about her body. She is about 5'8" now (I am nearly 5'10") and over the past year, has switched from feeling quite good about being tall to suddenly wanting to stop growing. Now.
Dd's weight is in the healthy range, but she frets because her belly is not flat like those of the models she sees on tv in their bikinis, even though she understands that most of them have gone without food and water for something like 24 hours before the photo shoot. Dd also worries because I am obese, so she thinks she has "the fat gene." I honestly don't believe I have a "fat gene;" I was thin all throughout my growing up years, but developed the habit of eating to fill emotional needs -- but remained thin in spite of that, probably due to my height, until it all caught up with me as an adult. I've explained all this to dd, and have also explained that she is a much more active person than I ever was. She currently goes out nearly every day, even in this cold weather, and takes a three-mile walk with our dog. She will also be starting basketball soon, and she participates in any group sports she has a chance to. We never push her to exercise, she does it on her own. I was never like this. I liked lying on the couch and reading.
So dd just doesn't need anyone trying to stress her out about weight or body size. Some of my mom's comments may be unintentional, such as when dd tried on some pants my mom offered her, and they were too tight, and my mom said she'd give them to so-and-so because she was thin. So-and-so is also just medium height and not as tall as my dd, which I pointed out to dd, but she was still upset by my mom's comment and felt like that meant she wasn't thin.
I think I need to help my dd not be so quick to take every comment like the above one as a jab, because I think it's just good practice to assume positive intent wherever possible. But I have had to talk with my mom, twice, about comments that she's made directly to my dd. On one occasion, she informed my dd that I used to be really thin, and didn't start gaining weight until I was older -- therefore dd was probably going to get heavy, too. I was very upset with my mom over this and made it clear that it was never to happen again.
Then on our next visit, while I was helping dd2 in the restroom, my mom told dd1 that if she was ever worried that she was gaining weight, she could just eat less. I think maybe my mom was somehow trying to make my dd feel better about her previous comment, but it really upset my dd. She told me about it after we left, and I called my mom later and told her that this had bothered dd and I really didn't want her saying things to her about her weight. It's been several months and it hasn't happened again.
On the one hand, I'd say that it's only necessary to confront Grandma if she's saying things directly to or about my children's weight -- but on the other hand, I can see how someone who's always going on about her own weight. and calling herself fat when she's really thin, can also indirectly hurt anyone who's larger than she is, who doesn't already feel good about his or her own weight. So I think I'd definitely avoid spending time with people who are weight obsessed for any reason. Of course, my own dd is rather weight obessed right now and I don't avoid spending time with her, but this thread has got me thinking that I really should introduce the idea to her that calling herself fat in front of others can end up hurting some people's feelings. For example, she has one friend who currently has a rather large belly, who could feel hurt by dd going on about her own much smaller belly.
This friend spent the night last weekend, and I discovered them lying on the floor of dd's bedroom, doing belly exercises with some of dd's weights on top of their bellies. I told them I didn't think this was a good idea and they might hurt themselves.